This application relates to the art of bonding hydrophobic thin films of amphiphilic molecules to substrate surfaces for protection against abrasion and stains. The invention is particularly applicable to the bonding of such films to magnesium fluoride antireflection coatings by way of a metal oxide primer film and will be described with particular reference thereto. However, it will be appreciated that the invention has broader aspects and can be used to bond hydrophobic films to other substrates having surface characteristics that are unfavorable to realization of a good bond with amphiphilic molecules.
The properties of magnesium fluoride include low chemical reactivity, a low refractive index, good scratch resistance, good weatherability and mild reaction to high energy radiation. Because of these properties, magnesium fluoride is used extensively as an antireflection and protective coating on surfaces of optical devices that are made of glass. The magnesium fluoride coating increases light transmission through the optical surface to which it is applied by minimizing light reflection. Examples of optical devices to which magnesium fluoride antireflection films are applied include eyewear lenses, sunglasses, binocular, microscope and telescope lenses, video terminal and television screens, liquid crystal displays, mirrors, prisms and optical surfaces of precision instruments.
Unfortunately, despite its excellent antireflection and protective properties. magnesium fluoride is not without faults as an antireflection coating. The surface of a magnesium fluoride antireflection coating is porous and behaves like a hydrophilic surface with an affinity for dirt and oils. This stains the surface, and distorts and reduces light transmission through the optical device. Frequent cleaning is required to preserve optical clarity, and this promotes the formation of scratches on the optical surface.
Staining of optical surfaces by dirt and oils can be minimized by applying a thin film of amphiphilic molecules that is hydrophobic and that seals the pores in the optical surface. The hydrobophic film provides a slippery and anti-stick surface that also is abrasion resistant without changing the color or reflection properties of the substrate to which it is applied. However, the surface of a magnesium fluoride antireflection coating is chemically inert, and this makes it extremely difficult to attach a hydrophobic film of amphiphilic molecules. The inventors are not aware of any published reports of a hydrophobic film of amphiphilic molecules attached to the surface of a magnesium fluoride antireflection coating.
Research has been reported on the coating of magnesium fluoride surfaces with metal oxides, metal oxo-acids and other metals. The coating is provided to change the antireflection properties of the magnesium fluoride layer or to activate the surface of the magnesium fluoride. Examples of publications that discuss such coatings, the disclosures of which are hereby incorporated herein by reference, are as follows: U.S. Pat. No. 3,034,916 issued May 15, 1962; Wojciechwska, M., Bull. Acad. Pol. Sci., Ser. Sci. Chim., 28, 237-47 (1980); 29, 549-62 (1981); Zukic, M. et al, Appl. Opt., 29, 4284-92 (1990); Catalan L. A. et al, Brit. J. Appl. Phys., 12,.499-502 (1961).
The prior methods for applying a coating of metal oxide to magnesium fluoride surfaces use very high temperatures, strong vacuums and require soaking and/or baking for a very long time. The prior methods do not control the thickness of the metal oxide coating, and a thick coating will undesirably change the reflection characteristics of an optical surface to which it is applied.
It would be desirable to be able to coat magnesium fluoride surfaces with a hydrophobic film of amphiphilic molecules to provide resistance against abrasion and staining.
In accordance with the present application, magnesium fluoride surfaces are coated with a hydrophobic film of amphiphilic molecules by way of a primer coating of a metal oxide having a surface that is hydrolyzed in the presence of airborne moisture to form hydroxy groups that react with the amphiphilic molecules.
In accordance with the present application, the surface of a magnesium fluoride substrate is provided with a thin film of a metal oxide by first immersing the substrate in a metal oxo-acid solution. The substrate is removed from the solution at a slow constant speed during which the solution solvent evaporates to leave a thin film of the metal oxo-acid on the substrate surface. The uniformity and thickness of the film is controlled by the metal oxo-acid concentration in the solution, by the evaporation rate of the solvent and by the speed at which the substrate is removed from the solution.
The substrate is then baked so that the metal oxo-acid self-condenses to a metal oxide on the magnesium fluoride surface, and also bonds to the magnesium fluoride surface by reacting with surface molecules to form a composite such as metal oxyfluoride.
Following baking, the substrate is cooled to ambient temperature and exposed to an ambient atmosphere that contains moisture. The metal oxide surface is hydrolyzed by airborne moisture to form active hydroxy groups.
The hydrolyzed surface of the metal oxide is then coated with a hydrophobic thin film of amphiphilic molecules in accordance with commonly assigned U.S. Pat. Nos. 5,078,791; 5,204,126; 5,219,654; and 5,766,698; the disclosures of which are hereby incorporated herein by reference. The amphiphilic molecules self-assemble on the surface of the metal oxide film and react with the hydroxy groups to form a chemical bond.
Metal oxide primer films in accordance with the present application are those having surfaces that are hydrolyzed by airborne moisture to form hydroxy groups that react with the amphiphilic molecules. Examples of suitable metal oxides include oxides of silicon, titanium, zirconium, hafnium, chromium and aluminum. The preferred oxide is silicon dioxide.
It is a principal object of the present invention to provide a procedure for bonding a hydrophobic thin film of amphiphilic molecules to the surface of a magnesium fluoride substrate.
It is a further object of the invention to provide such a procedure that can be carried out at relatively low temperatures, and that is economical and efficient.
It is another object of the invention to provide substrate surfaces with a primer film that is reactive with amphiphilic molecules.